Recent studies on the insertion loss of LiNbO.sub.3 devices have shown that optimum design will require control of the optical mode size and the degree of mode confinement in specific regions of the crystal. High mode confinement is necessary to minimize propagation and bending losses and maximize electric field overlap. A larger mode size near the edges increases the fiber to waveguide coupling efficiency at this interface. The desired degree of confinement and the mode size in the actual device region may vary with the application. Some devices require a different propagation constant (which varies with mode confinement) in each of two parallel waveguides which are separated by several microns.
The degree of mode confinement in any waveguide depends upon the physical size (cross section) of the waveguide and the magnitude of the refractive index difference between the core and the cladding. For Ti:LiNbO.sub.3 waveguides these parameters are moderately coupled due to fabrication restrictions. The geometrical dimensions can be varied by changing initial titanium strip width, diffusion temperature or diffusion time. The induced index difference can be varied by changing the diffusion parameters and by changing the initial titanium concentration. Because of the required dimensional tolerance, local control of the diffusion parameters by introducing a temperature gradient across a single crystal is difficult. Variations of the strip width and titanium concentration (or the combination of both) offer promising possibilities.
According to this invention, the amount of titanium available for diffusion is varied by removing metal in specific regions after a uniform layer has been deposited. In principle, this can be accomplished by ion milling but significant damage to the crystal results. Chemical etching using hydrofluoric acid has also been attempted but gives uncontrollable results. The thin TiO.sub.2 surface layer etches very slowly but, once etched, the underlying metal dissolves nearly instantly. We have found that the EDTA etch solution gives controllable etching. We have found also that this etching solution can be used effectively with photomasking operations to yield a selective process.